A mixer is an important block of radio frequency (RF) frontend circuits, and has been widely used in transceiver systems. The mixer is used to perform frequency shifting, i.e., to shift the carrier frequency of a desired signal from one frequency to another. For a transmitter, it shifts the desired signal from a low frequency band to a radio frequency band. This process is called as up-mixing or up-conversion. For a receiver, it shifts a desired signal from a radio frequency band to a low frequency band. This process is called as down-mixing or down-conversion.
The mixer performs signal mixing operations based on a local oscillating (LO) signal. In a direct conversion (DC) transceiver (TRX), for the down-conversion operation, the frequency of a received signal is the same as that of the LO signal, while in the up-conversion operation, the frequency of an output signal is the same as that of the LO signal. The LO signal is usually generated by dividing or multiplying the frequency of a voltage control oscillating (VCO) signal. The frequency of the LO signal fLO is N times larger or smaller than that of the VCO signal fVCO, i.e. fLO/fVCO=N or fLO/fVCO=1/N. Thus, the frequency of the desired signal is N times that of the VCO signal or vice versa, in the direct conversion architecture.
The frequency plan has a severe problem in the DC transceivers, i.e. VCO pulling. VCO pulling occurs when a power amplifier (PA) transmits the output signal (frf) with a frequency the same as the VCO frequency, or the PA harmonic is the same as the VCO frequency or even the VCO harmonic is the same PA signal. When the PA is integrated within the system this situation becomes worse because VCO is disturbed by the PA output and the transmitted signal may be totally destroyed. A common frequency plan in DC transceivers is fvco=2*flo=2*frf, so the quadrature LO signals could be generated by dividing the VCO signal by 2. In such architecture, VCO pulling is inevitable if VCO pulling is not handled carefully.
Thus, there is a need to a signal mixing method to resolve the aforesaid problem.